In German Patent Application No. DE 10 2004 018 221 A1, a method and a device are described in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine in front of an SCR catalyst. The reagent pressure is specified to a predefined reagent setpoint pressure as a function of a characteristic quantity. A performance quantity of the internal combustion engine and/or a characteristic quantity of the exhaust gas of the internal combustion engine can be utilized as characteristic quantity. The predefined reagent setpoint pressure is regulated within the framework of a closed-loop control in which the actual reagent pressure is detected by a reagent pressure sensor. It is possible to admix pressurized air to the reagent. The compressed air pressure may also be regulated within the framework of a closed-loop control, to a predefined setpoint compressed air pressure as a function of a characteristic quantity, the actual compressed air pressure being detected by a compressed air pressure sensor. A defect of at least one of the pressure sensors may cause reduced performance of the SCR catalyst with the result that non-purified exhaust gas may reach the environment.
In German Patent Application No. DE 10 2004 044 506 A1, a method and a device are described in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine in front of an SCR catalyst. The compressed air is guided via a non-return valve having an opening pressure. Provision is made for a diagnosis of the compressed air pressure, which begins at a starting instant with the closing of a compressed air regulation valve. At least at a second point in time it is checked whether the compressed air pressure corresponds to at least one lower threshold value, which at least approximately corresponds to the opening pressure, added to the ambient air pressure, of the non-return valve. A fault signal will be supplied if the condition is not satisfied.
In German Patent Application No. DE 101 59 849 A1, another method and device are described in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine in front of an SCR catalyst. Fuel is provided as reagent, which acts as reducing agent for the NO2 portion in the exhaust gas, in particular.